Sheets of glass having a thickness of not less than about 3 mm are tempered successfully according to present day regulations and practices, thereby to find use in automotive manufacture. One typical regulation is Reg. No. 43 of an agreement of the United Nations. This regulation calls for uniform conditions of approval, as well as a mutual recognition of approval of motor vehicle equipment and parts.
Typically, these regulations include a requirement that each sheet of glass, or more particularly the resultant fragments from an impacted sheet of glass, pass certain criteria. Thus, after an impact of a force sufficient to break the sheet of glass, the number of glass fragments within an area of 25 cm.sup.2 (5 cm.times.5 cm) at a distance greater than 7.5 cm from the point of impact should be between 40 and 350. No glass fragment, except possibly the glass fragments within the radius of 7.5 cm measured from the point of impact, should exceed 3 cm.sup.2 in surface area, and the glass fragments of elongated shape which may exist should not exceed 7.5 cm in length. Sheets of glass at high temperature, of a thickness not less than 3 mm and moving in a horizontal attitude, have been tempered to meet these conditions by a flow of air impinging on the sheet of glass at a speed on the order of about 200 m/s, with a flow blown on the sheet of glass that can approach 1200 Nm.sup.3 /m.sup.2 of glass/minute.
Consideration has been given to the possibility of increasing air flow to improve performance of the sheet of glass under considerations previously discussed. The technique has limitations. To this end, regardless of the dimension of the sheets of glass undergoing tempering, it is necessary to evacuate the air without altering or otherwise disturbing the blowing. As the air flow is increased, the requirement to evacuate the air increases, as does the likelihood that the air flow will be disturbed. Actually, in typical present-day installations about 60% of the total surface is taken up by the rolls of rollers which support the sheet of glass, the nozzles from which the air flow emanates and the equipment in general. In addition, about 5% of the total surface is occupied by incident blowing air. Thus, about 35% of the total surface is free surface for evacuation of air together with the calories or heat that is picked up from the sheet of glass. As may be appreciated, an increase in the volume of the air flow would require an increase in the size of the nozzles from which the air flow emanates. In addition, an increase in the volume of air flow would increase the total surface area occupied by incident air impinging on the glass sheet.
A surface area of about 35% of the total surface area is considered to constitute the lower limit for adequate evacuation of air, without alterations or otherwise disturbance of the blowing characteristics which will produce an inefficiency in cooling. Thus, an increase in the volume of the air flow would reduce the percentage below the lower limit.
An attempt to increase performance of the sheet of glass under impact has also centered about efforts to vary the air jets both in space and time. These variations include pulsation of jets and oscillation of the blowing devices. Also, it has been a consideration to overdensify the blowing in certain zones (alternating strong and weak tempering strips). While, essentially, it has been possible to obtain glasses without needles, that is, glasses without fragments longer than 7.5 cm with a higher degree of certainty, the result is only achieved at the cost of major complication of the apparatus which is used.
It is known that the heat exchange between air and the sheet of glass to be cooled is approximately proportional to the speed of movement of the flow of air. Therefore, in an effort to increase performance consideration has been given to increasing the feed pressure of the nozzles from which the flow of air emanates, thereby to reach greater air speeds. Feed pressures have been used to attain sonic, even supersonic speeds. This technique has made it possible to improve performance, albeit only slightly, and the improvement in performance has been realized in the tempering of sheets of glass of a thickness of 2.5 mm. However, the improvement in performance, once again, is realized only at the price of considerable cost in the expenditure of energy. What should be noted, however, is that improvement in performance has not always been realized in the tempering of sheets of glass of a thickness in the order of 2 mm, and the technique often results in a failure to satisfactorily meet regulations for use of the sheet of glass in automotive manufacture. And, it should be pointed out that the technique is not carried out under conditions that are economically satisfactory or compatible with industrial production using sheets of glass having a thickness of less than 3 mm.